Evaluation of the mechanical damage of recycled aggregate concrete by acoustic emission

The depletion of natural resources and the accumulation of construction and demolition waste have led the concrete industry to explore the use of recycled coarse aggregates (RCA) as a sustainable alternative to natural coarse aggregates (NCA). This paper presents an experimental investigation into the distinct and combined effects of two RCA granular classes, 4/8, 8/16, and their combination 4/16, on the concrete fracture process. Three-point bending tests, under crack mouth opening displacement (CMOD) control, are performed on concretes incorporating these RCA classes. Crack evolution is monitored using the acoustic emission (AE) technique. Recycled aggregate concretes (RAC) made with 100 % RCA of classes 4/8, 8/16, and 4/16 are compared with natural aggregate concrete (NAC) in terms of mechanical properties, fracture properties , acoustic activity, and characteristics of the fracture process zone (FPZ). The results show that the use of RCA of classes 4/8 and 8/16, either separately or in combination, achieves compressive and splitting tensile strengths comparable to those of NAC, despite higher porosity. However, this increased porosity reduces the modulus of elasticity of the RAC. The distinct use of these RCA classes maintains a fracture energy equivalent to that of NAC. However, their combination leads to a significant reduction in this energy. Furthermore, the separate use of RCA of classes 4/8 and 8/16 does not significantly impact the acoustic activity of RAC compared to NAC, while their combination generates an increase in this activity, indicating a more intense microcracking process, which aligns with the observed reduction in fracture energy. The incorporation of RCA, regardless of its class (4/8, 8/16, or 4/16), does not significantly influence the length of the FPZ but reduces its width. At the end of the test, the length of the FPZ of concrete with these RCA classes is approximately 67.78–68.44 mm (0.85 times the height of the ligament), and its width is about 26.33–27.66 mm (1.64–1.73 times the maximum size of the aggregates). The 4/16 class has a more pronounced impact on reducing the width of the FPZ of RAC than the 4/8 and 8/16 classes.